1. Field of the Invention
This invention relates to a guiding device for a needle thread used for enfolding or back tacking a thread chain by an overedge sewing machine.
2. Description of the Prior Art
When an overedge sewing operation by a conventional overedge sewing machine is finished, a thread chain is formed that extends from the fabric material. Where a two-needle overedge sewing machine is used, after each completion of the overedge sewing process, the thread chain is enfolded or back tacked between the fabric material and the looper threads to prevent the beginning or start edge of the fabric material from fraying. Such an enfolding or back tacking process is well known to those of skill in the art.
Prior art sewing machines have a throat plate, an inner chain-off finger, an outer chain-off finger and a drop point for a right-side needle. The fabric material has a thread chain enfolded or back tacked between the fabric and a looper thread. A thread-chain-holding-and-cutting-device clamps and cuts the thread chain when it is pulled in the direction of feeding.
It is a well-known practice to provide an inner chain-off finger at the throat plate in a prior art overedge sewing machine in order to prevent the fabric material from sinking or dropping stitches by the two needles, to prevent the skipping of stitches, and to make sure the fabric has a good hand or feel.
As used in the art, a "soft chain" means that the thread chain has a well-balanced needle thread and looper thread. When only a thread chain is formed, a thread tension adjuster in the prior art overedge sewing machine regulates the supply of looper thread, thus properly balancing the thread chain with the proper supply of threads, so that a soft chain is produced.
When overedging for each fabric material is finished, the fabric material is released from a presser foot by a pedal action. Then, the thread chain is pulled in the direction of feeding to release the twining threads around the inner chain-off finger and the outer chain-off finger.
The fabric material is next turned back or rotated to the operator's side, and the thread chain is inserted into the thread-chain-holding-and-cutting-device, and the thread chain is cut.
New fabric material is set with a presser foot in a pressing condition, and the stitching starts.
The thread chain is enfolded or back tacked into the seam of the new fabric material.
If the thread chain hangs at the inner chain-off finger, the thread chain is stitched by the needle, and in such case, the thread chain will not be enfolded or back tacked.
The prior art is sometimes effective in preventing the thread chain from hanging at the inner chain-off finger. However, the fabric material is frequently dropped during stitching, and the skipping of stitching is unavoidable. The overedging work is of inferior quality, too.
The mechanical construction of prior art devices is complicated and requires many parts with accurate machinings and is high in cost.